The present invention relates to the conversion of the state of polarization of an optical beam.
The state of polarization of an optical beam represents an important feature in many applications, in particular when measurements comprise polarization dependent components. Polarizers are typically inserted into the optical beam in order to provide a defined state of polarization. While optical signals with defined state of polarization will pass the polarizer, optical signals with other states of polarization will generally be absorbed or reflected. Disadvantageous in that solution, however, is that the optical power of the output beam after the polarizer can be significantly decreased with respect to the input beam. Further, the optical power of the output beam becomes a function of the state of polarization of the input beam.
A polarization converter for converting randomly polarized light to linearly polarized light is known from EP-A-431894. EP-A-489375 discloses a drum servo system. In EP-A-782028, an apparatus produces parallel beams with like polarization by aid of a polarization splitter/combiner, and the parallel beams are then subject to polarization dependent processing. U.S. Pat. No. 5,102,222 discloses a light wave polarization determination using a hybrid system. A polarimeter is described in U.S. Pat. No. 6,043,887.
It is an object of the present invention to reduce dependencies of a polarization corrected output beam on the variation of polarization over time of the input beam. The object is solved by the independent claims. Preferred embodiments are shown by the dependent claims.
According to the present invention, a polarization converter comprises a polarization dependent beam splitter splitting up an incoming optical beam into a first and a second beam, each with a defined state of polarization but different from each other. Preferably, one of the beams will be polarized horizontally while the other beam will be polarized vertically. A polarization adapter is provided to the second beam of the polarization dependent beam splitter. The polarization adapter converts the state of polarization from its input to its output in a way that the states of polarization of the first beam (from the polarization dependent splitter) and the output beam from the polarization adapter substantially match.
Each one of the two beams with substantially matching states of polarization might then optionally be provided to a polarizer with substantially the same intended state of polarization, in order to correct small deviations in the states of polarization. It goes without saying that the polarization orientation of the polarizer should match with the states of polarization of its input beams in order to reduce unwanted power consumption by the polarizer.
The polarization converter according to the present invention thus provides a first output beam (as the first beam from the polarization dependent beam splitter) and a second output beam (as the output from the polarization adapter with the second beam of the polarization dependent beam splitter as input thereof). The two output beams of the polarization converter are provided with substantially the same defined state of polarization. In case that the power consumption of the polarization dependent beam splitter, the polarization adapter, and the polarizer (if used) is neglectable (dependent on the characteristics of the components as well as their correct application), which should be the case in most applications, the sum of power of the two output beams of the polarization converter substantially equals the optical power of the input beam to the polarization converter.
Dependant on the application it might be useful to avoid interference effects between the two beams from the polarization dependent beam splitter as well as between the two output beams from the polarization converter. This can be made sure in that the two beams are spatially separated from each other.
In a preferred embodiment, the two output beams from the polarization converter are provided substantially in parallel and preferably with only a small distance between the two parallel output beams. The area of further optical components subjected to the two output beams from the polarization converter is preferably designed to be sufficiently large, so that the same optical component(s) can be applied for both output beams of the polarization converter. It is also possible to use two separate detectors (instead of one larger detector) and provide(e.g., electronically) a sum of individual photo currents from the two separate detectors.
The polarization adapter preferably comprises a xcex/2-plate which rotates the linear polarization by 90 degrees (and, for example, converts from parallel to perpendicular polarization).
In one embodiment, the optical power of the input is further detected in order to monitor a variation of the input power to the polarization converter. The detected input power can be used for power control purposes or to correct parasitic effects of the set-up, e.g., to calibrate the arrangement in terms of wavelength dependency of the detector response or the total set-up. This allows achieving a highest absolute power measurement accuracy.
Additional electrical circuits as well as some signal processing and related software functionality can be provided, dependent on the specific application, e.g., in order to process the signals of the detected photo currents. The invention can be partly supported by one or more suitable software programs, which can be stored on or otherwise provided by any kind of data carrier, and which might be executed in or by any suitable data processing unit. Such a program is preferably use dot provide algorithms for deriving the absolute power and wavelength information out of the individual detector signals and the calibration data.